A vehicle carrying Imran Khan, head of the Pakistan Tehreek-e-Insaf party, is surrounded by journalists while he arrives to attend a party meeting in Islamabad, Pakistan, Monday, Aug. 6, 2018. The party won the most parliament seats in last month's general elections and is expected to form a governing coalition later this month. (AP Photo/Anjum Naveed)

A Pakistani takes selfie with Imran Khan, center, head of the Pakistan Tehreek-e-Insaf party, as he leaves a party meeting in Islamabad, Pakistan, Monday, Aug. 6, 2018. The party won the most parliament seats in last month's general elections and is expected to form a governing coalition later this month. (AP Photo/Anjum Naveed)

A Pakistani takes selfie with Imran Khan, center, head of the Pakistan Tehreek-e-Insaf party, as he leaves a party meeting in Islamabad, Pakistan, Monday, Aug. 6, 2018. The party won the most parliament seats in last month's general elections and is expected to form a governing coalition later this month. (AP Photo/Anjum Naveed)

ISLAMABAD — Imran Khan's party on Monday officially nominated the cricket star-turned-politician to be Pakistan's next prime minister.

To assume office, Khan will face a vote in parliament — perhaps as early as Saturday — in which he will have to defeat a rival candidate fielded by the opposition.

Khan's Tahreek-e-Insaf party won the most parliament seats in last month's general elections — 115 — but fell short of a majority in the 342-seat assembly, requiring it to form a coalition. Many lawmakers who won as independents in the July 25 vote have joined his coalition.

Tahreek-e-Insaf leader Arif Alvi, a lawmaker elected from Karachi, said Khan was formally nominated at a party meeting in Islamabad on Monday. He was unanimously endorsed, said Shah Mahmood Qureshi, a former foreign minister and party deputy leader.

But Khan is likely to face tough opposition from the Pakistan Muslim League of ousted Prime Minister Nawaz Sharif and other parties, which allege vote-rigging in last month's elections. Sharif is currently appealing a 10-year prison sentence for corruption. The party is led by his brother, Shabaz Sharif, who was also nominated for prime minister on Monday.

Qureshi insisted the opposition would not be able to undermine the new government and quoted Khan as saying during Monday's meeting that all they can do is "create a rumpus."

The Pakistan Muslim League held its meeting in the eastern city of Lahore to discuss the formation of the provincial government in Punjab. It's relying on assembly members who won seats as independents to side with it in the 371-seat provincial assembly. The party won 129 seats there.

Khan's party, which won 123 seats in Punjab, also aspires to form the provincial government, saying it has won over many independents.

Maryam Aurangzeb, a spokeswoman and former information minister, said the Pakistan Muslim League will hold a protest against poll rigging in front of the election oversight body on Aug 8. "Election results were manipulated in favor of a party and we will expose all these tactics before the nation," she said.

On Monday, the Tehreek Labbeik party, led by radical cleric Khadim Rizvi, held a protest rally in the eastern city of Lahore against the alleged vote rigging. Rizvi said the protests would continue until the 'fake results' are cancelled.

His party, which strongly supports a controversial law mandating death for blasphemy, won two provincial seats in the elections.